The present invention relates to active matrix type liquid crystal display units (AM-LCD units) driven by thin film transistors (TFTs), and to processes concerning the manufacture of these AM-LCD units.
The market for thin film transistor-driven liquid crystal display units (TFT-LCD units) is expanding as these imaging display units are further improved in thickness, weight, and resolution. In recent years, along with the tendency of TFT-LCD units to increase in screen size and in resolution, the need for reduced resistance values relating to signal lines and their terminals and improved production yields has been increasingly stringent. Reduction in production costs is also being demanded.
For LCD units of the active matrix type, scanning lines extending in a horizontal (x-axial) direction and arranged in parallel in a vertical (y-axial) direction, and data lines extending in a vertical (y-axial) direction and arranged in parallel in a horizontal (x-axial) direction, are formed on the liquid crystal side of one of a pair of transparent substrates opposed to each other via liquid crystals, and the rectangular areas surrounded by these signal lines are used as pixel areas. Each pixel area is provided with both thin film transistors driven by scanning signals supplied from the scanning signal lines on one side, and pixel electrodes driven by data signals supplied from the data lines on another side via the foregoing thin film transistors. Each pixel is driven by scanning signals or data signals, and these scanning signals or data signals are supplied through the terminals of the scanning lines or data lines extending to the outer area of the display section that is formed as a set of pixel areas.
Each scanning line and each data line are sheathed with either an insulating film that also functions as the gate insulator of a thin film transistor, or a passivation film that also functions to avoid direct contact with the liquid crystals of said thin film transistor. The terminals of these signal lines are exposed by provision of holes in the insulating film or passivation film. It is known that the occurrence of so-called xe2x80x9celectrocorrosionxe2x80x9d can be prevented by sheathing said exposed surfaces with poly-crystalline indium tin oxide (p-ITO), one type of indium tin oxide consisting mainly of a poly-crystalline phase.
Each scanning line is required to be low in electrical resistance, to have sufficient dry-etching resistance so as not to be lost by dry etching intended to provide holes in its insulating film or passivation film, and to come into proper contact with p-ITO at its terminals. For these reasons, scanning signal lines are made of materials such as a chromium-molybdenum (CrMo) alloy/chromium (Cr) film, where the slash xe2x80x9c/xe2x80x9d denotes lamination and the left and right sides of the slash denote the upper and lower layers, respectively, of the lamination. (Hereinafter, the slash and both sides thereof mean the same.) In addition to satisfying the above-mentioned requirements, each data line is further required to come into proper contact with amorphous silicon since the source electrode and drain electrode of the thin film transistor are to be formed at the same time. Also, it is desirable that each data line be capable of being provided with wet-etching to ensure the selectability between the gate insulator and amorphous silicon during etching. For these reasons, data lines are made of materials such as a CrMo alloy/Cr film. The use of these wiring materials enables the amount of side etching to be reduced below 1 xcexcm and the cross section of the wiring to be processed into a tapered shape. These processing characteristics, in turn, enable the obtainment of thin film transistors provided with excellent characteristics, including a channel length of 7 xcexcm or less, and can be applied to pixel electrode processing for an In-Plane Switching(IPS) type of LCD unit.
Signal wiring in an LCD unit is formed so that a voltage is applied by the driver chips mounted in the LCD unit, and the electrical connection resistances at the connections (contacts or terminals) of these driver chips are also required to be reduced. In recent years, significant decreases in the areas of the contacts, coupled with further improvements in display resolution, have resulted in a tendency towards increased connection resistance. For example, the method of mounting drivers in an LCD unit is shifting from the conventional TCP (Tape Carrier Package) method to the COG (Chip-on-Glass) method, and decreases in the areas of the contacts in the case of this method are significant. The terminals of the signal lines sheathed with the above-mentioned p-ITO satisfy the need for reduced electrical connection resistance. Also, p-ITO satisfies the functional requirements of transparent pixel electrodes, and the transparent pixel electrodes and the terminal sheathing mentioned above are formed at the same time during the manufacturing processes.
Further progress of LCD units in terms of dimensions and resolution results in a more stringent need for reduction in the electrical resistance of scanning lines, in particular. Accordingly, materials that include aluminum (Al) or Al alloy will be used, instead of the CrMo alloy/Cr film mentioned above. (Hereinafter, such signal lines are also referred to as aluminum wiring). Also, wiring structure in which Mo or the like is included in the upper layer of Al or Al alloy is usually employed so that proper contact with p-ITO can be obtained at the terminals of the scanning lines. For example, molybdenum-zirconium (MoZr)/Al-alloyed films whose upper layer is made of an MoZr alloy that includes Zr at a rate from 2.6 to 23 weight % enables favorable etching into the forward tapered cross section of the wiring, and gives sufficient dry-etching resistance to the MoZr layer required when the insulating film on the MoZr/Al-alloyed films is provided with through-hole processing.
It was verified, however, that the use of scanning signal lines made of aluminum wiring and the use of transparent pixel electrodes and terminal sheathing made of p-ITO or the like induce the following inconvenience: to etch p-ITO, it is absolutely necessary to use a strong halogen acid solution, such as a hydrobromic acid (HBr) solution, and these solutions permeate defects in the insulating film sheathing the signal line and also functioning as the gate insulator of the thin film transistor, and defects in the passive film functioning as an insulating film to avoid direct contact with the crystals of that thin film transistor. Thus, the signal line is disconnected, and this, in turn, reduces production yields and increases production costs.
One method that can be used to prevent the signal line from being disconnected is by using either amorphous ITO (a-ITO) or indium-zinc oxide (IZO), instead of p-ITO, as transparent pixel electrodes and terminals, and further using oxalic acid ((COOH)2) as the etchant for processing these electrodes and the terminal sheathing, since the rate of dissolution of aluminum and aluminum alloy in oxalic acid is sufficiently low that signal line disconnections are not caused.
The selection of a-ITO or IZO, however, will give significantly high connection resistance to the driving circuits and turn out to be a long way from achieving the intended purpose of reducing the electrical contact resistance. FIG. 14 is a graph showing the contact resistance values of p-ITO, a-ITO, and IZO materials. In FIG. 14, the horizontal axis denotes changes in the needle pressure applied to each material, and the vertical axis denotes changes in the contact resistance between each material and the needle at that time. For LCD units, the bump pressures driving chips applied to the signal lines of the terminals are considered to almost stay within range A of the graph, and, in this case, it can be seen that, in terms of reduction in contact resistance, using p-ITO is much more effective than using a-ITO or IZO.
Such increases in connection resistance due to the selection of a-ITO or IZO can be suppressed by removing a portion or all of the a-ITO or IZO sheathing from the terminals of the signal lines and thus exposing the metallic portions of the terminal wiring. Incidentally, FIG. 15 shows data on the contact resistance between various materials and an anisotropic conductive film (ACF). Direct contact with MoZr/Al alloyed films and CrMo alloy/Cr film, compared with that of a-ITO or IZO sheathed terminals, reduces the contact resistance with respect to ACF. Compared with the applicability to the TCP method, however, the value of the contact resistance between a CrMo/Cr and ACF does not decrease to such an extent that this value can be applied to the COG method in which the area of the contact section is greatly reduced. Contact resistance between ACF and the MoZr/Al-alloyed film used for the scanning lines is low enough to permit application to the COG method.
The problem that the present invention is to solve, therefore, is how to create, in view of the factors discussed above, data line, source electrode, and drain electrode film materials that satisfy the following requirements:
(1) The materials must have dry-etching resistance and not be lost during through-hole processing of the insulating film.
(2) The materials must have a specific resistance value equivalent to, or less than that of, a CrMo/Cr film.
(3) The materials must permit etching with an etchant that will scarcely etch SiN, which is the material used for the insulating film.
(4) In order to obtain thin film transistors having excellent characteristics, the materials must enable the amount of side etching to be reduced below 1 xcexcm and the channel lengths of the thin film transistors to be reduced to 7 xcexcm or less.
(5) The materials must enable the cross section of the electrodes or signal lines or to be tapered and the pixel electrodes of an IPS type LCD unit to be processed.
(6) Contact resistance with respect to ACF must be low enough to permit application to the COG method.
(7) The number of film layers must not exceed two.
It is obvious from the above description that except for requirement (6) above, the CrMo/Cr film mentioned above can be applied. Also, except for requirement (7) above, an Mo/Al/Mo-alloyed triple-layer film is most likely to satisfy all other requirements. However, since triple-layer film deposition by spattering is a time-consuming operation, this reduces productivity significantly. Also, a large-scale deposition apparatus is required and this increases plant investment. For the present invention, therefore, we have searched for suitable single-layer or double-layer film materials.
The film materials that satisfy requirements (1) to (7) above are a molybdenum-alloyed film having a chromium content from 2 weight % or more, but up to 5 weight %, and a molybdenum-alloyed film lamination consisting of a first electroconductive film and a second electroconductive film, with the first electroconductive film having a chromium content from 2 weight % or more, but up to 5 weight %, and the second electroconductive film being provided above the first electroconductive film and having a smaller chromium content than the first electroconductive film. The use of either such film material for the source electrode and drain electrode of a thin film transistor enables this transistor to have a high performance, and/or even more effects can be obtained by using either material for a data line. In short, it is effective to use these materials for the source electrode and drain electrode of a thin film transistor and/or for data line. Materials data proving this are described below.
The high-melting-point metals that can satisfy requirement (3) above are molybdenum (Mo) and chromium (Cr). Also, it has been considered from the data of FIG. 15 that Mo-based alloys are lower than Cr-based alloys in terms of contact resistance with respect to ACF. We have therefore searched for candidate materials within the range of Mo-based alloys.
FIG. 16 shows investigation results relating to the effects of added elements on the dry-etching resistance of an Mo alloy. It can be seen from this figure that dry-etching resistance can be enhanced by adding Cr, Zr and Hf to Mo. FIG. 17 represents the relationship between the SF6 plasma dry-etching rate of an Mo alloy and its added elements (Cr, Zr, and Hf). Incidentally, the dry-etching rate of SiN, which is the insulating film material, is 7.3 nm/sec. To allow for its use as a material for the data lines, it is desirable that the selection ratio of Mo alloy etching with respect to SiN etching should be at least about seven times as great, and the dry-etching rate of an Mo alloy is required to not exceed 0.13 nm/sec. The composition of the Mo alloy satisfying these requirements is: MoCr (Crxe2x89xa71.5 weight %), MoZr (Zrxe2x89xa75.0 weight %), MoHf (Hfxe2x89xa78.0 weight %).
FIGS. 18(a) and 18(b) show the relationship between the resistivity of an Mo alloy and the quantities of added elements (Cr, Zr, and Hf). FIG. 18(a) shows data that was obtained by adding Cr, and FIG. 18(b) shows data that was obtained by adding Zr and Hf. Incidentally, the resistivities of CrMo/Cr films are about 220 nxcexa9xc2x7m. Although each element increases in resistivity with increases in the amount of added elements, when the resistivity of each element is compared on the basis of the minimum quantity of added elements that satisfies the dry-etching resistance mentioned above, the resistivities of an MoZr film having a Zr content of 5.0 weight % and an MoHf film having an Hf content of 8.0 weight % are about 300 nxcexa9xc2x7m, which is well above the resistivities of CrMo/Cr films. An MoCr film with a Cr content of 1.5 weight %, however, has a resistivity of about 180 nxcexa9xc2x7m, which is lower than that of the CrMo/Cr film, and even the resistivity of an MoCr film having a Cr content less than 5.0 weight % is only about 240 nxcexa9xc2x7m, which is about 10% higher than the resistivity of the CrMo/Cr film. In terms of resistivity, therefore, it can be safely said that the MoCr(Crxe2x89xa65 weight %) film is equivalent to CrMo/Cr films. Hence, we have searched for candidate materials within the range of MoCr (1.5xe2x89xa6Crxe2x89xa65 weight %).
FIG. 19 shows the relationship between the rate of MoCr alloy etching with a mixture of phosphoric acid, nitric acid, and acetic acid, and the quantity of the added element (Cr). The etching rate has its own appropriate range. For example, if the etching rate is as great as over 10 nm/sec, the amount of etching cannot be appropriately controlled, since the etching time required becomes too short. Conversely, for example, if the etching time exceeds two minutes, normal patterning may not be achievable since the soundness of the resist cannot be obtained. In addition, the machine cycle time tends to increase, which is not preferable in terms of production efficiency. In the range of MoCr (1.5xe2x89xa6Crxe2x89xa65 weight %), etching rates fall within an 8.0-1.7 nm/sec range, and for the reasons mentioned above, this etching rate range is approximately appropriate. In the range of MoCr (Crxe2x89xa75 weight %), however, the etching time required for a standard wiring film thickness of 200 nm exceeds two minutes. Etching in the range of MoCr (Crxe2x89xa75 weight %), therefore, is not preferable for the reasons mentioned above.
FIG. 20 shows changes in the amount of MoCr side etching with a mixture of phosphoric acid, nitric acid, and acetic acid. The amount of side etching decreases as the Cr content in the MoCr alloy is increased. In consideration of photolithographic resolution and developing process, to obtain thin film transistors 7 xcexcm or less in channel length, the amount of side etching needs to be controlled to below 1 xcexcm, as will be described later with reference to an embodiment of the present invention. The Cr content in the MoCr alloy, therefore, needs to be 2 weight % or more, and in view of its fluctuations, this value should desirably be 2.5 weight % or more. Even for an MoCr-alloyed double-layer film lamination whose lower layer is made of a 180 nm-thickness Mo-2.5 weight % Cr alloy and whose upper layer is made of a 20 nm-thickness Mo-1.6 weight % Cr alloy, the amount of side etching was almost the same as that of the Mo-2.5 weight % Cr alloy.
FIGS. 21(a) to 21(c) show examples of the cross-sectional shapes of the wiring which was obtained from MoCr alloy etching with a mixture of phosphoric acid, nitric acid, and acetic acid. When the Cr content in the MoCr alloy is 1.6 weight % as shown in FIG. 21(a), the wiring obtained takes almost a vertical cross-sectional shape. When the Cr content in the MoCr alloy is 2.5 weight % as shown in FIG. 21(b), a shape of about 30 degrees in taper angle is obtained. For LCD units of the IPS type, it is known that, as will be described later with reference to an embodiment of the present invention, when the wiring of the pixel electrodes has a vertical cross-sectional shape, there occurs a display nonuniformity ascribable to the rubbing process for alignment layers. For this purpose, therefore, Mo-1.6 weight % Cr is inappropriate. As shown in FIG. 21(c), wiring with a cross-sectional shape of about 30 degrees in taper angle was also obtained in the MoCr-alloyed double-layer film lamination whose lower layer is made of a 180 nm-thickness Mo-2.5 weight % CR alloy and whose upper layer is made of a 20 nm-thickness Mo-1.6 weight % CR alloy. Although, at this taper angle, the Mo-2.5 weight % Cr single-layer film creates a more significant in-plane nonuniformity than the film lamination shown in FIG. 21(c), the Mo-2.5 weight % Cr single-layer film can also be put into practical use without a problem.
FIG. 15 also shows the contact resistance values of Mo-2.5 weight % Cr alloyed films with respect to ACF. These values are much smaller than those of a CrMo/Cr film, and are not inferior to a p-ITO sheathed film. It is therefore possible for COG mounting to be implemented using the terminals where an Mo-2.5 weight % Cr alloyed film and ACF are brought into direct contact.
It can be synthetically judged from the above materials data that the film materials that satisfy requirements (1) to (7) are a molybdenum-alloyed film having a chromium content from 2 weight % or more, but up to 5 weight %, and a molybdenum-alloyed film lamination consisting of a first electroconductive film and a second electroconductive film, with the first electroconductive film having a chromium content from 2 weight % or more, but up to 5 weight %, and the second electroconductive film being provided above the first electroconductive film and being smaller in chromium content than the first electroconductive film. The use of either such film material enables the production of an LCD unit which is 7 xcexcm or less in terms of thin film transistor channel length, and if the LCD unit is of the IPS type, this unit can be kept free from display nonuniformity ascribable to the film rubbing processes.
According to the present invention, a liquid crystal display (LCD) unit having excellent characteristics can be provided.